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The circulatory system is all about distributing oxygen around the body. The mighty heart — which never rests as long as we live — the 60,000 miles worth of blood vessels, and blood itself, all come down to this: Every cell in our bodies needs a fresh supply of oxygen every few minutes, or it will die. And so will we.

The human heart

The heart is at the center of the circulatory system, a hollow organ composed of muscle and connective tissue. In humans, the heart has four chambers — two atria or "entrances," and two ventricles or "bellies" — and weighs less than a pound.

The heart beats optimally about 70 times a minute throughout our lives, beginning within three weeks after conception, for a total of about 3 billion pulses in a lifetime of 80 years.

The heart pumps blood to the lungs, where it picks up its cargo of oxygen, and then on to the rest of the body. Valves in the heart and the blood vessels ensure that blood travels in one direction only, away from the heart in the arteries, and toward the heart in the veins.

Red blood cells travel single-file through the capillaries, the fine vessels that connect the arteries and the veins, to deliver oxygen and other nutrients to the cells. Here the blood takes on waste, especially carbon dioxide, which it will deposit in the lungs for expulsion into the air.

A septum in the middle of heart keeps waste-filled blood returning from its journey through the body separate from oxygenated blood fresh the lungs.